Page 145 - A Comprehensive Guide to Solar Energy Systems

144 A COmPREHEnSIVE GuIdE TO SOlAR EnERGy SySTEmS

to the deployment of CSTP plants. nevertheless, the proven ability of CSTP plants to store
energy is something that PV plants can’t offer.
most of the current R + d efforts related to CST technologies are guided by this need to
reduce costs and also the need to improvement overall efficiency.
Concerning PTC technology, the two main R + d technology trends being
investigated are: new working fluids for the solar field; and new collector designs. The
replacement of the thermal oil currently used as working fluid in the solar field by
silicone-based oils would not only allow higher steam temperatures for the PCS (and
therefore, higher plant efficiencies), but also less operation and maintenance costs
due to the lower environmental hazard of silicone oils. The use of molten salt, water/
steam and compressed gases as working fluids for PTC is also under investigation and
the advantages and disadvantages of these three innovative working fluids have been
analysed by Zarza [10].
The development of new PTC designs with higher concentration ratios is also being
researched, as it would create a temperature increase in the solar field without increasing
the thermal losses. Achievement of higher concentration ratio requires mirrors with better
optical properties as well as sun tracking systems with a higher accuracy.
Concerning central receiver technology, the main technology trends are related to new
working fluids to achieve higher temperatures and therefore higher efficiencies. In some
cases, the new working fluids investigated demand a re-design of the main plant compo-
nents (i.e., central receiver, thermal storage system, and PCS). new working fluids under
investigation are:

– molten salts suitable for temperatures higher than 600°C
– falling particles
– supercritical CO 2 , and
– atmospheric or compressed air
molten salt currently used in CSTP plants is composed of a binary mixture of potas-
−1
sium nitrate (40%) and sodium nitrate (60%). This mixture is cheap (about €1 kg ) and
it has good thermal stability up to 600°C. new salt compositions are required to increase
the thermal stability up to 700°C or more, while keeping a moderate melting point and low
viscosity in the liquid phase.
The use of small solid particles, with a diameter of 1–2 mm, carried by an air stream is
also being investigated as an option to increase the working temperature to temperatures
above 600°C and hence to increase the efficiency of CSTP plants [17]. The solid particles
heated in the receiver can then be stored in a thermally insulated vessel to be used later
on when solar radiation is not available. The main challenges when using a falling particle
receiver are the erosion produced by the friction of the particles inside the piping and ves-
sels, and the stability of the particles after many heating-cooling cycles. Transport of the
particles from one component to another is also a technical constraint because conven-
tional blowers or pumps can’t be used for this purpose.